This invention relates to a slit mechanism for use in a monochromator, in which the slit width is changed by selectively using a plurality of slits each having its own fixed width.
In a known slit mechanism wherein a plurality of slits having a fixed width are exchanged for selective use, all the slits have the same height which is determined in accordance with the height of the slit of the smallest width. This determination, however, has no reasonable theoretical base. A slit of a narrow width is used to obtain a sufficient resolution of wavelength. If a slit has too great a height, the curvature of the spectral image of the slit along the height thereof reduces the monochromaticity of the output light under measurement. Therefore, the height of the slit is determined with a view to attaining a required wavelength resolution. A slit of a large width is selected to obtain a high intensity. In this case, however, the resolution is reduced as the slit width is broadened, and there is no design factor by which the height of each slit is determined, so that the heights of all the slits are determined simply to conform to that of the slit of the narrowest width.
The intensity of the output light emerging from a monochromator is propositional to the square of the width of the exit slit. In particular, if the entrance and exit slits have a height h and a width w, the intensity of the output light coming out of the exit slit is given as Khw.sup.2 where K is a constant. Therefore, if the width of the slit is doubled with the height thereof remaining unchanged, the intensity of the output light is quadrupled, and if the height of the slit of the double width is halved, the intensity of the light from the slit is halved, that is, reduced to half the original intensity.
On the other hand, the surface of a diffraction grating considerably scatters the light incidient thereon, and some of the scattered light reaches the exit slit as stray light, which obstructs the measurement of spectra. In a monochromator, in addition to the above-mentioned scattered light from the grating the scattered light from various other optical elements such as collimating mirrors and the inner wall surface of the casing causes stray light. The amount of the stray light is proportional to the amount of light incident on the grating and other elements and also to the area of the exit slit, that is, the square of the slit width w and the square of the slit height h and is given as K'h.sup.2 w.sup.2 where K' is a constant. Therefore, the ratio of the intensity of the stray light to that of the output light, that is, the stray light level P is given as P=K'h.sup.2 w.sup.2 /Khw.sup.2 =(K'/K)h.
If the slit width is doubled, the intensity of the spectrum obtained and the intensity of the stray light are both quadrupled, that is, four times the intensity obtained with the original slit width, so that the stray light level P remains unchanged. However, if the height of both slits is halved, the intensity of the spectrum is halved or becomes half the intensity obtained with the original slit height since 4.times.1/2=2 while the intensity of the stray light is reduced to one quarter or becomes equal to the intensity obtained with the original slit height since 4.times.(1/2).sup.2 =1, so that the stray light level P is reduced to one half the level with the original slit height.
In conventional monochromators in which the slit width is variable, no consideration was given to stray light and the slit height is kept unchanged.